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Full text of "Mathematical And Physical Papers - Iii"

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356          ON THE  CHANGE  OF  REFRANGIBILITY  OF   LIGHT.
regularly refracted beam of violet, and a scattered beam of white light. Of these the latter would be insignificant compared with the former, were it not that the illuminating power of the colours belonging to the middle of the spectrum is so very much greater than that of the violet. When the dispersed beam was analysed by a prism, it would be decomposed into a violet beam of definite refrangibility, followed by a dark interval, and then a broad band containing the colours of the brighter part of the spectrum in their natural order. This is what is constantly scon in cases of true dispersion; but the polarization of the beam, and its behaviour under the action of absorbing media, would reveal the counterfeit character of the dispersion.
On the Colours of Natural Bodies.
174. By this expression I mean to include only the colours to which it is usually applied, namely, those of leaves, flowers, paints, dyed articles, &c., which form the great mass of the colours that fall under our observation. I do not refer to colours due fo refraction, such as those of the rainbow, or to diffraction, such as those of the coronas seen about the sun and moon, or to interference, such as those seen in the clear wings of small Hies, or to the colours which accompany specular reflexion, which last m*e usually but slight, though sometimes pretty intense.
In some few instances, as for example in the case of fluor-spar, various salts of peroxide of uranium, acid solutions of disulpha.to of quinine, &c., colours are observed, sufficiently strong to arrest, attention, which have a remarkable and hitherto unsuspected origin. But I am not now speaking of colours arising from a change of refrangibility in the incident light. In the vast- majority of cases these colours are far too feeble to form any sensible, portion of the whole colour observed. The colours which dyed articles give out under the influence of the highly refrangible rays usually agree more or less nearly with those of which such substances commonly appear, and it is possible that the colour arising from a, change of refrangibility may contribute in some slight degree to the brilliancy of the tint observed. If, however, (lie effect be sensible I am persuaded that it is but slight; and very brilliant colours may be produced without a change of refrangibility, as for